The present invention relates to jet propulsion, and in particular to a propulsion assembly capable of operating both in an atmospheric mode and in an exoatmospheric mode.
In the field of chemical jet thrusters, a distinction is drawn between rocket engines in which thrust is generated by the expansion of gas produced by a chemical reaction of one or more propellants, and air-breathing thrusters in which the thrust is generated by expanding gas produced by combustion of a fuel in air.
Rocket engines are normally non-air-breathing, and they have the particular advantage of being capable of operating even outside the Earth's atmosphere. Nevertheless, in order to feed them, it is necessary to carry a greater weight of propellant. By using ambient air for propulsion, air-breathing thrusters can carry a smaller weight, but they cease to operate as soon as the ambient density of oxygen drops below a certain threshold. In addition, among air-breathing jet thrusters, turbojets present a very high degree of mechanical complexity and are very difficult to adapt to hypersonic speeds. Ram jets, in which ambient air is compressed by the relative speed of the thruster, are simpler and are better adapted to very high speeds than turbojets, but they need initially to be propelled by other means up to a starting speed.
As means for performing this initial acceleration of ram jets, proposals have been made in particular to use rocket engines. In particular, using the concept known as “air-augmented” combustion, a rocket engine and a stream of air are combined in such a manner that in an air-breathing mode, combustion of at least one propellant of the rocket engine is increased by the air flowing in the air flow. A first example of an air-augmented combustion propulsion assembly was the solid propellant GNOM concept put forward in the Soviet Union in the 1960s. Another example was the American liquid propellant GTX project, which nevertheless did not advance beyond the study stage.
In patent application FR 2 933 744, another propulsion assembly of that type is disclosed, comprising a channel defined by an inner wall and an outer wall, and presenting an inlet opening and an outlet opening, together with a plurality of masts, each mast connecting together said inner and outer walls and each having rocket engines oriented in an axial direction. In that propulsion assembly, the rocket engines act as ejectors driving a stream of air to flow along the channel, and by injecting a fuel-rich gas into that stream of air, post-combustion is facilitated downstream from the masts.
Nevertheless, that propulsion assembly is designed solely for propulsion in the Earth's atmosphere, and it is not suitable for exoatmospheric propulsion. In order to enable propulsion gas to expand into the vacuum of space, exoatmospheric jet thrusters normally require nozzles of very large outlet section, which is difficult to make compatible with the configuration of the FR 2 933 744 propulsion assembly. In addition, cooling the throat of such a nozzle can present major technical obstacles.